The present invention relates to electron multipliers. More specifically, the present invention is related to electron multipliers used as detectors for time-of-flight mass spectrometry.
Electron multipliers are often utilized as detectors for time-of-flight mass spectrometry. There are two types of electron multipliers: discrete dynode electron multipliers and continuous dynode electron multipliers. Discrete dynode multipliers generally consist of a cathode; a series of dynodes, shaped plates or assemblies of plates; and an anode connected together by a chain of resistors. A high voltage is applied across the chain to create a potential difference between each pair of dynodes that drives secondary electrons down the dynode chain to the anode.
In an electron multiplier, an ion or other particle striking the cathode will produce secondary electrons that are accelerated to the first dynode. Upon striking the first dynode, these electrons generate another set of secondary electrons which are in turn accelerated to the second dynode, and so on through the multiplier. When the potential difference between a pair of dynodes is large enough each electron striking a dynode will, on average, produce more than one secondary electron. The average number of secondary electrons per primary electron produced at a particular dynode is the gain of that stage of the electron multiplier. The gain of the entire electron multiplier is the product of the gain at every stage from the cathode to the last dynode. Increasing the voltage applied to the electron multiplier typically increases the voltage between dynodes, increasing the gain of each stage, thereby increasing the gain of the entire multiplier. Typical electron multipliers have 10-30 stages, operate with an applied voltage of 1000-5000V, and are capable of producing gains larger than 105.
Discrete dynode multipliers are commonly used for the detection of particles such as photons, ions or neutral molecules. Because of the very large gains possible with electron multipliers it is possible to detect, with some efficiency, the arrival of single particles that have enough energy to cause the generation of secondary electrons at the conversion surface of the electron multiplier. At the same time, it is possible for an electron multiplier to behave linearly with incident signals corresponding to over a thousand particles arriving simultaneously. In addition to this instantaneous dynamic range, electron multipliers typically have response times less than a few nanoseconds and noise levels corresponding to less than a few incident particles per minute. Together these characteristics make electron multipliers useful for measuring particle fluxes from a few particles per minute to hundreds of particles per nanosecond.
FIG. 1 is a typical wiring diagram 100 for a simple electron multiplier. An external voltage source needs to be connected to the electron multiplier in such a way that the cathode 102 and each succeeding multiplier stage are correctly biased with respect to one another. Because electrons must be accelerated through the electron multiplier, the first dynode 104 is held at a potential higher than the cathode 102 and each succeeding dynode 106-116 is held at a potential higher than the preceding dynode. For efficient operation, the potentials applied across the first few stages of the electron multiplier are often several times the potentials applied to the stages in the middle of the multiplier. The interstage voltages of an electron multiplier may be supplied by individual voltage sources such as batteries or power supplies, or, as is more common, by a small number of voltage sources 122 and a network of resistors that forms a multi-stage voltage divider 120.
Because of the multiplying function of an electron multiplier, each dynode will source more electrons than the preceding dynode. Thus, the voltage sources near the anode 118 must supply more current than those earlier in the chain. Because the ion fluxes measured with electron multipliers are generally pulsed, the extra current for the dynodes near the anode 118 can be supplied with capacitors 124. These capacitors reduce the change in voltage between dynodes caused by the loss of electrons during multiplication (amplification) of an input signal and then recharge through the bias network 120 during periods where there is little or no input signal.
As long as the output of the multiplier is in fixed proportion to the input signal, the electron multiplier is said to be operating linearly. For input signals near the upper end of the linear range of an electron multiplier, the electron multiplier can only maintain the large output signal until the loss of electrons from the dynodes and their associated capacitors causes the voltage on the dynodes to change significantly; this, in turn, causes the gain of the multiplier to change. At this point, the electron multiplier is said to be entering saturation. If the large input signal continues, the gain of the electron multiplier will continue to decrease until the output signal is small enough that it can be supplied continuously. At this point, the electron multiplier can be said to be completely saturated.
To recover from a saturating event, the capacitance associated with the dynodes of the electron multiplier must recharge. This recharge typically occurs through the resistors of the bias network. Since the bias network of electron multipliers generally have impedances of about 107 ohms and the dynodes have capacitances near 10−11 F the recharging of the dynode capacitance occurs with a characteristic time of approximately 10−4 s. Extra capacitance added as a charge reservoir can dramatically increase this time. For example, 10 nF of extra capacitance will increase the characteristic recharge time to 0.1 s. These are very long times when compared to the typical few ns width of the pulses produced by the electron multiplier. During this recharging time the multiplier does not have the gain or linearity of a multiplier with a fully charged dynode chain.
The long time required to recover from charge depletion induced non-linearity limits the utility of electron multipliers in situations where small signals-of-interest follow large signals that can drive the multiplier into a charge depleted state. Matrix assisted laser desorption/ionization time of flight mass spectrometry (“MALDI-TOFMS”) is such an application. In MALDI-TOFMS, the ions-of-interest follow, in time, a large matrix signal that can drive the electron multiplier into charge depletion and prevent the efficient detection of ions for a substantial amount of time after the matrix signal has ended.
One way of addressing the charge depletion is to design an electron multiplier with more capacitance in the dynode chain. An example of an implementation of this solution was presented at the 2002 meeting of American Society for Mass Spectrometry (“ASMS”) in Orlando Fla. (Kevin L. Hunter, Dick Stresau, Wayne Sheils, “Influence of capacitance networks on the pulse dynamic range and recovery time of time-of-flight detectors”). The extra capacitance added to each dynode allowed the electron multiplier to source much larger output currents before entering charge depletion. FIG. 2 shows the circuit diagram 200 of an electron multiplier modified to have capacitors 226-244 connected with each of the dynodes in the dynode chain.
The additional capacitance defers the onset of charge depletion, but, since the detector can only source a fixed amount of charge over its lifetime, the additional capacitance and the larger possible output current can result in a substantially shortened detector lifetime. Initial results indicate that the lifetime of such a detector can be as short as several days when used for MALDI-TOFMS. Another disadvantage of the additional capacitance is a substantially increased recovery time for the electron multiplier after saturation.
There is therefore a need for an improved electron multiplier that does not suffer from the above-mentioned shortcomings.